It works!  Mostly.

This is far from tested.  But the very preliminary testing is
encouraging.  Balances are correct.  Starts immediately on load.
Backups appear to be correct.  Addresses are all displayed properly.

A few things I've noted in testing:

- Zero-conf tx show up in wallet list balance but not ledger until you do
  a refresh of the tx ledger
- Zero-conf disappear when a new block shows up that doesn't include
  your zero-conf tx
- Importing/sweeping/etc still asks if you want to rescan.
- I'm sure billions of little other things